In vehicles, predominantly permanently energized, d.c. current motors are used for the powering of heating, ventilation and air conditioning blowers. To achieve the necessary variation in the output of the blower motor wire-wound or printed series power resistors are used.
The power dissipated by the series resistors amounts to, depending on the characteristics of the blower, up to 25% of the electric power input into the blower motor, when the motor is run at its maximum output. The series resistors are disposed, preferably, in the path of the air current of the blower, to dissipate the heat generated by the series resistors.
In case of heavy, blocked or short circuited blower motors, the power dissipation of the series connected resistors increases greatly, while the ability of the system to cool the resistors decreases at the same time. The short circuit current limited by the series resistor, however, is generally not sufficient to trigger the fuse of the blower circuit.
It is known in the art to protect series resistors from damage, destruction or fire, by using high temperature resistant materials from which to make the series resistor and from which to make the materials surrounding the series resistor in the circuit, as well as to protect the place in the circuit surrounding the resistors against penetration by and deposit of combustible foreign bodies. Furthermore, greatly enlarged series resistors are used, which are designed for their ability to cope with short circuits, in case no air current is present to cool the resistors. In these circumstances, however, the series resistors cannot be housed, in most cases, in compactly built heating and blower housings made of thermoplastic plastics. Beside the additional construction space, additional masking and cabling will be necessary.
It is also known to equip series resistors with an excess-temperature switch, which switches the voltage supply directly to the load, thus causing the safety fuse of the blower circuit to respond.
Furthermore, guard switches with reconnecting locks are known, which interrupt the circuit at the series resistor because of excess current and/or excess temperature. These measures require a considerable expenditure of time for the adjusting and checking of the temperature and/or current-dependent switching function. Because of the possibility of the contacts becoming contaminated or coming loose over many years of inoperation, these extra switches may not provide a reliable safety mechanism, or with regard to contact welding.
In case of other known circuits, additional melting or soldering safety devices to protect the resistor circuit have been provided. They are also partly integrated directly into the series resistor. When activated, these safety devices are irreversibly altered; thus their safety function cannot be checked.
In case of expensive, electronic protective circuits, relays or power semiconductors are provided which interrupt the circuit. The turnoff signal results from a temperature, current, voltage or resistance measurement or from a combination of these values. Such protective circuits require sensors which sense the indicated values of a particular variable to be measured, electronic utilization of the indicated value, and a switching member; such protective circuits are expensive and require additional construction space and additional cablings.